The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present invention.
Many systems or devices requiring an electrical power supply also incorporate a backup power supply to avoid disruptions in the operation of the system or device. This can be a critical requirement in the computing arena where, for example, a single power supply may be used for a bank of computers such as servers. The backup power supply can provide power during short gaps that may occur in the event of a temporary failure or fluctuation in the primary power supply or during a transfer from one primary power supply to another primary power supply.
Ideally, the backup power supply should be capable of storing power for an extended period of time, be able to provide substantially constant backup power for a required length of time (at least thirty seconds), and have the ability to recharge quickly.
Batteries have often been used for this backup purpose in many applications. However, batteries have several disadvantages. Primary type batteries tend to naturally deplete over time and lose their ability to store power, resulting in a shelf life of as little as approximately six months. Secondary type rechargeable batteries generally recharge at a much slower rate than their discharge rate. The long re-charging period can be a great disadvantage in systems that may experience frequent power failures or transfers. Secondary batteries generally have a limited life span that may be dictated by the number of discharge/recharge cycles. A typical secondary battery may be limited to approximately 2,000 cycles. Both types of batteries have unpredictable lifetimes, in other words, it is difficult to predict when a battery will reach the end of its useful voltage or life. Batteries, such useful life can typically be predicted to only within about 40%, in other words, if a battery is expected to provide a minimum voltage for 10 hours, in reality such voltage might occur at any time between 6 hours and 14 hours.
Ultracapacitors are capable of storing energy and provide the advantage of rapid rechargeability. Ultracapacitors can generally recharge at the same rate as the discharge rate. However, the output voltage from an ultracapacitor generally declines rapidly, thereby reducing the available time for back up power. As an example, FIG. 1 illustrates the decline in the output voltage of an exemplary bank of ultracapacitors. The desired output voltage is generally a range of voltages. In this example, the desired range is indicated by the maximum (100%) voltage level and a minimum rated voltage level. A bank of ultracapacitors at full initial charge declines to the minimum voltage level rapidly (approximately 30 seconds in the example). By so rapidly reaching the minimum rated voltage, much of the energy stored in the ultracapacitors is rendered unusable. For example, only 45% of the energy stored in the ultracapacitors may have been used when the output voltage declines to the minimum rated voltage.
It is desirable to have a power source that provides a long storage life, constant output voltage for an extended length of time, ability to recharge quickly, and predictable useful voltage.